After discussing the cell, it would only be fitting to talk about how the cell gets nutrients in and waste out. The cell is surrounded by the cell membrane, which acts as the gate for everything that needs to pass into and out of the cell.

Particles will move freely across the cell membrane until equilibrium is reached. Equilibrium means that the concentration (amount) of a substance is the same on both sides. When the concentration of a substance is greater on the outside of the cell membrane, the particles will move across the membrane from the higher side to the lower side until both sides are equal. That process, where particles move from an area of higher concentration to an area of lower concentration, is called diffusion. In the case of water, diffusion gets a special name: osmosis. Osmosis is the diffusion of water.

Diffusion and osmosis occur naturally with the random movement of particles, so they cost the cell no energy.

Some particles are too big to properly move through the cell membrane, so the particles are helped across the membrane. Facilitated diffusion is when particles move across a cell membrane through a protein channel. Because the particles are still moving with the concentration, not against it, facilitated concentration costs the cell no energy. However, sometimes the opposite of facilitated diffusion occurs. Active transport is the process where a cell uses energy to move particles from a lower concentration to a higher concentration. Again proteins are used to make this process work, but instead of protein channels, they’re called protein pumps.

Below you’ll find an animated gif that shows the process of diffusion of 2 blue particles, which we’ll call water, and the facilitated diffusion of 1 green particle, which we’ll call glucose. The animation was originally done in Flash, but I couldn’t host a SWF or even a movie file for free, so I had to convert it to a GIF. Maybe if these videos catch on I’ll do the YouTube thing, but for now I hope this works.